The present invention relates to a method of producing thin sheets which are stretched over a frame and have a planar and smooth surface with a low defect density and to the use of such sheets for the production of X-ray masks as defined in the preamble of claim 1.
The periodical Solid State Technology (SSTEAP 27 (9)), September 1984, pages 192-199, illustrates and describes, on page 194, in an article by A. R. Shimkunas, entitled "Advances in X-Ray Mask Technology," a method of manufacturing an X-ray mask blank of boron nitride. In this method, a silicon wafer is initially coated with boron nitride on all sides. Then the central region of the rear side is etched away until the silicon is exposed and the wafer is connected with a Pyrex ring. After being coated with polyimide and metallization of the frontal face, the silicon is etched away from the exposed rear side so that a mask blank results in which a membrane is stretched which can be processed further into an X-ray mask. However, this method can be employed only for such sheet or membrane materials which are not attacked by the etching medium. Titanium, for example, is also etched away by conventional etching media for silicon.
The periodical, Journal of the Electrochemical Society, Volume 128, No. 5, May, 1981, pages 1116-1120, discloses in an article by W. D. Buckley, J. F. Nesler and H. Windischmann, entitled "X-Ray Lithography Mask Technology," on pages 1117 and 1118, a method of producing a titanium membrane in which absorber structures of gold are applied to the membrane by galvanoplastic means. To accomplish this, a glass substrate is initially provided with a thin layer of gold onto which the titanium is then applied. The titanium membrane is connected with a ring and is mechanically separated from the glass substrate. Then, the membrane is stretched and connected with a supporting ring. The free stretching of the membrane, however, is a difficultly managed procedure. Moreover, in this method, the gold layer remains adhering to the titanium membrane so that the X-ray contrast with reference to the absorber structures is reduced. A similar method is disclosed in European published application EP-B1-0,104,685 corresponding to U.S. Pat. No. 4,701,391.